Boosting viscosity sensitivity of magnetic particle imaging using selection field gradients

In magnetic particle imaging (MPI), selection field (SF) gradients are utilized to form a field-free point (FFP) in space, such that only the magnetic nanoparticles (MNPs) in the vicinity of the FFP respond to the applied drive field (DF) and contribute to the received signal. While the relaxation behavior of MNPs adversely affects image quality by reducing signal intensity and causing blurring, it also provides MPI with functional imaging capabilities, such as viscosity and temperature mapping. This work investigates the effects of SF gradients on the relaxation behavior of the MNPs using an in-house magnetic particle spectrometer (MPS) setup equipped with an additional DC electromagnet SF coil, which switches the MPS setup into an MPI system. The results reveal that the presence of SF gradients boosts the viscosity sensitivity of MPI, and that the MPI signal can be sensitized to viscosity even at high DF frequencies and amplitudes if sufficiently large SF gradients are applied.